The present invention relates to composite articles, and more particularly to mica-containing ceramic matrix composite articles reinforced with inorganic fibers and exhibiting desirable tough fracture behavior.
Fiber-reinforced ceramic matrix composites comprising glass-ceramic matrices are well known. Each of U.S. Pat. Nos. 4,615,987, 4,589,900, and 4,755,798, for example, discloses silicon carbide fiber and/or whisker reinforced glass-ceramic composites wherein the matrix consists of an alkaline earth aluminosilicate glass-ceramic composition.
Prospective uses for fiber-reinforced ceramic matrix composites such as described in these and other prior patents and literature include use as a structural element in high temperature environments such as heat engines. For these and other applications, the materials to be employed must exhibit good strength and toughness at ambient as well as at elevated temperatures.
An important problem which has been identified in silicon carbide reinforced ceramic matrix composites, particularly after exposure to temperatures in the 800.degree.-1000.degree. C. temperature regime in an oxidizing environment at stresses above the microcrack stress point, is that of high temperature embrittlement. Hence, instead of exhibiting high toughness and strength after exposure to temperatures in the operation ranges desired, these materials become brittle and subject to sudden catastrophic breakage, rather than more gradual failure as typical of the original material. While the exact mechanism of embrittlement has not been fully explained, oxidative deterioration of the fiber-matrix interface is the probable cause. See, for example, R. L. Stewart et al., "Fracture of SiC Fiber/Glass-Ceramic Composites as a Function of Temperature," in Fracture Mechanics of Ceramics, R. C. Bradt et al. Ed., Volume 7, pages 33-51, Plenum (New York) 1986.
It is known to provide coatings on fiber reinforcement to be incorporated in composite materials in order to modify the behavior of the materials or the fibers therein. For example, boron nitride coatings have been applied to silicon carbide fibers or other fibers to be incorporated in ceramic matrix materials such as SiO.sub.2, ZrO.sub.2, mullite, and cordierite as disclosed in U.S. Pat. No. 4,642,271. The objective of coating the fibers in that patent is to attempt to preserve the room-temperature strength and toughness of the composites at elevated temperatures.
Other coating systems and coating matrix combinations are also known. U.S. Pat. No. 4,397,901, for example, describes a composite article and method for making it wherein a woven or non-woven fiber substrate, typically composed of carbon fibers, is provided with successive coatings of pyrolytic carbon, diffused silicon, and silicon carbide to provide a composite article resistant to corrosive conditions. U.S. Pat. No. 4,405,685 describes a similar coating system for carbon fibers wherein an inner coating consisting of a mixture of carbon and a selected metal carbide, in combination with an outer coating consisting solely of the metal carbide, are described. This dual coating system is intended to provide enhanced fiber protection for fibers to be embedded in ceramic or particularly metal matrix materials.
U.S. Pat. No. 4,605,588 discloses a process for providing a boron nitride surface coating on ceramic fibers such as aluminoborosilicate fibers. The boron nitride surface coating is reportedly effective to reduce reaction bonding of the fiber to the glass or ceramic matrix, thereby preserving the necessary toughening mechanisms in the composite system. U.S. Pat. No. 3,869,335 describes metal coated fibers and metal-glass coated fibers which can be incorporated into glass matrix materials to provide products which exhibit higher ductility than conventional products.
Attempts have also been made to modify the behavior of the ceramic matrix materials used in these composites to mitigate harmful effects of the matrix on the fibers. Thus, for example, U.S. Pat. No. 4,485,179 suggests the introduction of a tantalum or niobium additive to a ceramic matrix composite containing silicon carbide fibers to reduce fiber-matrix interactions therein.
In the physical testing of ceramic matrix composites embrittled during or subsequent to high temperature exposure, decreases in fracture toughness accompanied by changes in the fracture habit of the material are typically observed. Thus the predominant fracture mode changes from one characterized by fiber pullout from the matrix to one wherein woody fracture or, ultimately, brittle fracture occurs. Woody fracture surfaces display some crack propagation parallel to the stress axis, indicating localized shear failure but without fibrous pullout. Brittle fracture surfaces display merely planar fracture surfaces as the composite exhibits no plastic deformation.
The onset of brittle fracture behavior in these composites typically occurs in conjunction with significant reductions in fracture toughness. One indicator of this reduced toughness is a drop in the extent of strain or sample elongation observed above the so-called microcrack yield point of the material, as hereinafter more fully described.
Among the factors believed to influence fracture toughness are fiber debonding and fiber pullout behavior, including the degree of frictional resistance to fiber pullout from the matrix, as well as crack deflection occurring in the matrix and at the fiber-matrix interface. It may be postulated that modifications to the matrix and/or to the fiber reinforcement which would preserve fiber debonding or pullout at elevated temperatures would significantly aid in the development of composites exhibiting good high temperature toughness. This is part of the motivation for the development of coatings designed to preserve the desirable pullout behavior of silicon carbide fibers at higher and higher temperatures.
It is therefore a principal object of the present invention to provide fiber-reinforced ceramic matrix composites exhibiting improved fiber pullout behavior and/or improved toughness retention at elevated temperatures in an oxidizing atmosphere.
It is a further object of the invention to provide a process for treating inorganic reinforcement fibers for ceramic matrix composites to improve the fiber pullout characteristics of the fibers at elevated temperatures.
Other objects and advantages of the invention will become apparent from the following description thereof.